Do Honda 2-motor hybrids clutch-in the engine during heavy acceleration?

The Claim

In an article about the EPA rating of the 2018 Honda Accord Hybrid the following statement was made (emphasis added):

“During heavy acceleration or high-speed cruising, a clutch locks the engine and motor together to power the front wheels—a “parallel hybrid” drive—that is more efficient.” — GreenCarReports

2018 Honda Clarity Plug-In Hybrid

The Facts

Typically during heavy acceleration, a clutch unlocks the engine from the main electric motor and wheels in order to run in a series hybrid mode so the gas engine’s effective gearing can be adjusted to maximize power and torque output in addition to efficiency.

Essentially the same Honda Two-Motor hybrid system is used in the Honda Clarity Plug-In Hybrid but tweaked with a modestly down-sized engine and a larger and externally rechargeable battery. It all ends up with the same 212 total system horsepower as the Accord Hybrid but with 47 miles of electric range and about 10 percent lower gasoline mpg.

Standard disclaimer: this is an Auto Correction article. Topics are chosen not to point out a mistake in a specific article but rather to discuss a recurring point of confusion in media coverage or among EV enthusiasts. Getting all of the details correct in every article is very difficult (if not impossible), especially on a deadline.

Discussion

This hybrid system design was introduced in the 2014 Honda Accord Hybrid and remains largely unchanged up through 2018 except for incremental tweaks.

1. EV Drive is when the gas engine is off and the car is running entirely on battery power.

2. Hybrid Drive is a series hybrid mode where the gas engine is spinning a generator and the car is being pushed only by another electric motor (there is no mechanical link between the gas engine and the wheels).

3. Engine Drive is a parallel hybrid mode where the gas engine is directly clutched via a fixed gear ratio to the wheels and an electric motor can assist the engine using battery power or “steal” some engine power by regenerating energy into the battery.

In the section describing the system control strategy, the Honda SAE paper says:

During low-load situation such as launching or city driving, “EV Drive” is mainly selected. Then, driving mode is switched to “Hybrid Drive” for the acceleration during normal-load or heavy-load situation….

If the system efficiency can be enhanced by transferring engine output directly to the wheels like high speed cruising, the engine drive clutch is engaged in order to select “Engine Drive” mode.

In other words, the car typically operates in or switches to Hybrid Drive (series hybrid mode) under heavy-load conditions like strong acceleration or maintaining speed on a steep uphill climb.

People tend to find this use of the series hybrid mode counter-intuitive. Why not use Engine Drive where the engine is directly connected to the wheels? The reason is that the fixed gear ratio used in Engine Drive is an “overdrive” gear. It’s fixed gear ratio is actually equivalent to the 6th gear in the manual transmission version of the conventional Honda Accord.

If you were driving in 6th gear in a heavy-load condition you would normally downshift to put the engine into a better gear ratio for maximum torque generation. Honda’s hybrid system can’t do that since it has only a single fixed gear in Engine Drive so instead it switches to Hybrid Drive.

As can be seen in the graph above, the Hybrid Drive and Engine Drive modes each have zones of operation at a particular vehicle speed and torque output in which they can either assist the gas engine by using the battery and motor or can regeneratively brake or “steal” some energy from the output of the gas engine in order to keep the battery at a good state of charge.

“In the third mode, Engine Drive, a computer gently engages a wet clutch, connecting the engine to the front axle’s differential. In the interests of efficiency, no battery charging is allowed here; the engine runs quietly at low rpm with its computer-operated throttle barely open. On our drive through wine country, we found this third mode elusive and impossible to maintain for more than a few seconds at 65 to 70 mph. We suspect it exists mainly for use in EPA highway-mileage tests.” — Car and Driver

Actually, Engine Drive is the typical engine mode when cruising along a flat or gently hilly highway at 65 to 70 mph when the gas engine is running. Contrary to the claim that Engine Drive does not allow battery charging, it is definitely allowed as shown on the earlier graph from Honda.

In discussing this, the SAE paper says:

Although “Engine Drive” mode is mainly utilized during high speed cruise,….engine load adjustment by charging or discharging the high voltage battery is also performed in this mode to fix engine operating points in highly efficient area…

“Hybrid Drive” is selected if strong acceleration is needed and “EV Drive” is selected during slight acceleration or deceleration. As mentioned before, intermittent operation between “Engine Drive” and “EV Drive” is performed during slight acceleration or cruise in a high speed situation.